Platelet activation and aggregation are involved in unstable angina and acute myocardial infarction, in reocclusion following thrombolytic therapy and angioplasty, in transient ischemic attacks and in a variety of other vaso-occlusive disorders. When a blood vessel is damaged either by acute intervention such as angioplasty, or, more chronically, by the pathophysiological processes of atherosclerosis, platelets are activated to adhere to the disrupted surface and to each other. This activation, adherence and aggregation may lead to occlusive thrombus formation in the lumen of the blood vessel.
The final obligatory step in platelet aggregation is the binding of fibrinogen to an activated membrane-bound glycoprotein complex, GP IIb/IIIa (.alpha..sub.II .beta..sub.3). Platelet activators such as thrombin, collagen, epinephrine or ADP, are generated as an outgrowth of tissue damage. During activation, GP IIb/IIIa undergoes changes in conformation that result in exposure of occult binding sites for fibrinogen. There are six putative recognition sites within fibrinogen for GP IIb/IIIa and thus fibrinogen can potentially act as a hexavalent ligand to crossing GP IIb/IIIa molecules on adjacent platelets. A deficiency in either fibrinogen or GP IIb/IIIa prevents normal platelet aggregation regardless of the agonist used to activate the platelets. Since the binding of fibrinogen to its platelet receptor is an obligatory component of normal aggregation, GP IIb/IIIa is an attractive target for an antithrombotic agent.
Results from clinical trials of GP IIb/IIIa inhibitors support this hypothesis. The monoclonal antibody 7E3, which blocks the GP IIb/IIIa receptor, has been shown to be an effective therapy for the high risk angioplasty population. It is used as an adjunct to percutaneous transluminal coronary angioplasty or atherectomy for the prevention of acute cardiac ischemic complications in patients at high risk for abrupt closure of the treated coronary vessel.
A study reported in The New England Journal of Medicine vol. 330, No. 14, pp. 956-961 (1994) showed a decrease from 12.8% to 8.3% in the combined endpoints of death, non-fatal MI and need for urgent revascularization with fibrinogen receptor blockade. This benefit was at the expense of some additional risk of bleeding, with the need for transfusion increasing from 3% to 6%, and the incidence of patients with decreased hematocrit increasing from 7% to 15%. 7E3 was added to the standard regime of heparin and aspirin thus leaving few hemostatic control mechanisms intact. The clinical benefits of this drug could be seen at 6 months.
Many other studies have shown that blocking the GPIIb/IIIa receptor will stop platelet aggregation induced by all of the agonists and thus prevent thrombus formation but leave platelet adhesion relatively intact. The 7E3 monoclonal antibody is described in Coller et al. Ann. NY Acad. Sci. 1991; 614: 193-213; and Coller et al. J. Clin Invest. 1985; 76: 101-108. Others have used agents based on the RGD sequence, including snake venom proteins, small peptides, and peptidomimetics (Cook et al. Drugs of Future 1994; 19: 135-159; and Cox et al. Medicinal Research Reviews 1994; 14: 195-228). Integrilin is a cyclic peptide that is based on the KGD sequence in the snake venom protein barbourin (Cook et al. ibid.; and Cox et al. ibid.). It inhibits ligand binding to GPIIb/IIIa but has very little effect on ligand binding to .alpha..sub.v .beta..sub.3. Among the non-peptide compounds are RO44-9883 and MK-383, which are administered intravenously, and are also selective for GPIIb/IIIa (Cook et al. ibid.; and Cox et al. ibid.). Orally active agents include SC54684, a prodrug with high oral bioavailability, and R043-8857, GR144053, and DMP728, which are themselves the active inhibitors (Cook et al. ibid.; and Cox et al. ibid.). Literally thousands of other compounds have been synthesized in an attempt to obtain optimal potency, metabolic stability, receptor specificity, and favorable intravascular survival. Despite variations in these compounds, virtually of all of them retain the basic charge relations of the RGD sequence with a positive charge separated from a negative charge by approximately 10-20 .ANG. (Cook et al. ibid.; and Cox et al. ibid.).
The results of the 7E3 study support the hypothesis that blockade of GPIIb/IIIa receptors is more effective than aspirin in preventing platelet thrombi, even in the presence of heparin. They also support the hypothesis that platelet-dependent thrombi frequently contribute significantly to the development of ischemic complications after PTCA, even when minor mechanical dissections are present.
Heparin is a commonly used anticoagulant. Unfractionated heparin unpredictably binds electrostatically to proteins in addition to tethering to tighten weak antithrombin III:thrombin interactions. Low molecular weight heparin tends to bind in a more predictable pharmacodynamic fashion and is therefore more effective than unfractionated heparin in reducing clot formation. Cohen et al., N Enyl J Med 1997; 337:447-52 compares low molecular weight heparin to unfractionated heparin for treatment of unstable coronary artery disease. Patients received 1 mg of enoxaparin (low molecular weight heparin) per kg body weight subcutaneously twice daily, or continuous intravenous unfractionated heparin at a dose adjusted according to the activated partial-thromboplastin time, targeting the activated partial-thromboplastin time at 55 to 85 seconds. All patients received 100 to 325 mg of oral aspirin daily. Results showed that antithrombotic therapy with enoxaparin plus aspirin was more effective than unfractionated heparin plus aspirin in reducing the incidence of ischemic events in patients with unstable angina or non-Q-wave myocardial infarction in the early phase. The benefit with enoxaparin was achieved with an increase in minor but not major bleeding.